Manufacture of oleum



H: F. MERRIAM ET AL July 31, 1951 MANUFACTURE OF OLEUM 5 Sheets-Sheet 1 Filed March l1, 1944 '7059 ggf/Vy;

July 3i, 1951 H. F. MERRIAM ET AL, 256224@ MANUFACTURE oF OLEUM ENVENTOR5. #67223/ E/Wewzam 729m /mymf? 5 ATT@ R N EY 5 Sheets-Shee 4 l H.F. MERRIAM ETAL MANUFACTURE 0F OLEUM ,0 mmf@ \\\\gmwwmw/w f 0 m M00/ www A 6, 0 H m w .w w 0 M M ,wmwmmwwwwmwwwmmwmf w m fr.; m Tw. w. w M w M w M M M w Ow,

July 3E, 195i Filed March 11, 1944 July 31, 1951 H. F. MERRIAM E-rAl.

MANUFACTURE oF OLEUM Filed March 11, 1944 atented July 31, T951 Henry F. Merriam, West Orange, N. J., and Tom Cummings, Brooklyn, N. Y., assignors to Allied Chemical & Dye Corporation, a corporation of New York Application March 11, 1944, Serial No. 526,108

6 Claims. (Cl. 23-167) This invention relates to improvements in processes for manufacture of oleum. l

O'leulm of commercial strengths isY made customarily by contacting oleum with a gas containing-S03y under conditions such as to eiiect absorption of S03 while adding acid of lower strength to maintain constant composition in thev contacting zone. A

`lrincipal object of this invention is to provide procedure byv practice of which it is possible to greatly increase eiiiciency of S03 gas absorption oleum liquors. y'

The invention, its objects and the afforded 'ad-- vantages will appear from the following description taken in connection with the accompanying drawings. Fig. 1 shows diagrammatically apparatus 4suitable' for use in production of oleum in accordance with known practice. Figs. 2 and 3 diag-rammatically indicate apparatus in which oleum may be made by utilizing principles of the present invention. Figs. 4, 5 and 6 are illustrative and comparative charts, purposes of which will liereinafterappear. Y

Referring toV Figs. 1, 2 and 3, oleum production towers l0, ill and I2 are of design and construction generally the same as oleum pro-duction towers known in the art, and are ordinarily bricklined steel shells filled with adequate packing. Eachtower is provided a-t the top with a gas outlet, and aliquor inlet and distributing head, and at thebottom with a gas inlet and a liquor outlet.

CoolersY lfd, i5 and I6 are connected to receive oleum runningout of the bottom of the respective oleum production towers. Sulfuric/ acid productiony towers. I8: and Hi (Figs. l and 2)-, built thev same. as absorption towers' customarily used in manufacture of. sulfuric acid from S03-, discharge'stronge. g. 99% sulfuric acid into-coolers 2| fand-1 22. It will bev understood that each of the oleum and sulfuric acidv towers is provided with .itsown liquor circulating system4 including the respective coolers mentioned along with suitable pipe ,connections indicated diagrammatical-ly on the drawings, together with pumps and control valves, etc.,; not shown, all arranged. to maintain circulation thru the Various' towers ofsuitable quantities of oleum or sulfuric acid kept at temperatures required to accomplish the. desired S03 absorption in any particular tower.

YIncoming S03 from a contact plant enters' the systems vof Figs. l-Sthru conduits 25; 26 and, 2-1, passes in seriesv thru multi-tower units of Figs. Land-21, `ande tail gasescontaining substantially 11o-@S03 leavefthe'tops. of sulfuric acidi production tQWers iS-.and t9; and theftop of oleum production tower I2 and are discharged into the plant stacks. Gas movement thru the systems is indicated on the drawings by dotted lines. The diluent used, e. g. wateror sulfuric acid of suitable H2SO4 strength, is fed into the circulating circuits of towers I8, I9 and i2 thru inletsk 29, 3i) and 3l respectively. Liquor flow thru the systems, indicated on the drawing by solid lines, is generally countercurrent to gas flow. The make of oleum production. towers it, ii and l2, drawn off thru lines 33,V 34 and 35, constitutes the products.

The nature vof the invention, and the characteristics of the prior art difliculties which the present improvements largely offset, may be best understood by preliminary discussion of manufacture oi oleum as currently practiced. In effecting absorption of S03 gas in oleum liquors, three factors which must be taken into account are: first, the S03 concentration of the sulfur trioxide gas fed to the oleum system; second, the rapid rise in per cent S03 by volume present in vapor over oleum as temperature and/or strength of oleum increase; and third, the relatively high freezing points `of oleum products oil strengths within the more usual commercial ranges. The S03 gas employed is derived from commercial sources, e. g. the converted gas of ,an S03 contact pliant, and since such gases of most commercial installations frequently contain about 8.5% or less S03, it will be seen that the S03 gases utilized in manufacture yof oleum arer of relatively low S03 concentrations. Further, for a given set of operating conditions, freezing point oi the oleum product and the per cent S03 in vapor over such oleum are relatively fixed.

n Fig. 1 indicates a representative prior art commercial two-tower system for making oleum of say ,40% strength. From the legends on the drawing, over-all operating conditions will be appar ent. For illustrative .purposeathe incoming gas entering the bottom of tower iii may have-fan S03 concentration of 8.1%; `Oleum of 46% strength freezes at about 94 F. Hence', all of the liquor in tower lil, cooler it, and in the associatedfliquor circulating system must be kept at temperature above 94 F., and toY aircrd a practical operating safety factor, usual practice is such as to keep temperature: atleast about 10 F. above 94 F. Per cent SO3- by volume in vapor over 40% oleum (e. g. the per .cent S03 by volume in the vapor leavingthetopfof. tower it) is about 4.8 at F., that is, it temperatureA of gas leaving the top of tower lili: is held! at about 105 F;, such gas contains about 4.8% S03 by volume. Accordingly, itywillbev seen'fthat becausezof the relatively low strength of available incoming S03, high freezing point of the oleum product, the fairly high percentage of S03 in vapor over 40% oleum, and the desirability of operating at temperature as low as possible in order to promote S03 absorption, it is necessary to operate tower I within very narrow temperature limits in order to effect reasonably good absorption of S03 and keep the size of the equipment at a minimum. When making 40% oleum product, using an 8.1% S03 gas, and tower exit temperatures of about 1057F., only about 42% of the total S03 entering the bottom of tower I9 is absorbed. In this connection, curves 6.0, 4I, 42, 43, and 4d. of the chart of Fig. 5 show the per cent S03 absorption eiciencies at temperatures of about 9er-130o F., when making 40% oleum from S03 gases produced by catalytic conversion of sulfur dioxide gases containing 7, 8, 9, 10 and 11% S02 by volurne respectively, such gases when oxidized to sulfur trioxide under eicient contacting conditions forming sulfur trioxide gases containing about 7.0, 8.1, 9.1, 10.2, and 11.3% S03 by volume respectively. It will be noted that at practical operating temperatures of about 105 F., these absorption emciencies range from about 32% to about 60 Further, it will be noted that the system of Fig. 1 produces excess 99% sulfuric acid. The reason is that, because of the high S03 content of the gas entering tower I8, a large amount of diluent sulfuric acid must be used to clean all of the S03 out of the gas stream and more 99% HzSOi is produced than can be converted to oleum in tower I0. Thus, the system of Fig. 1 is not balanced which term designates operation such that all S03 of incoming gas is converted to oleum of the desired strength, and no S03 is taken out of the system as intermediate strength oleum or as sulfuric acid. While the unbalanced disadvantage of Fig. 1 may be remedied by use of an intermediate strength oleum tower between towers I0 and I8, such procedure involves substantial capital and operating outlay. Hence, in order to maintain balance, use maximum amounts for fortification acid, and produce maximum amounts of oleum, a three tower system is often employed for making 40% oleum. We have discovered primarily that if, for a given set of oleum-S03 gas contacting conditions, the contacting operation is carried out in such a way that the oleum liquor in the contacting zone contains certain amounts of HNOs, the per cent S03 by volume in Vapor over the oleum in such zone is substantially reduced as compared with the per cent S03 by Volume in vapor over the same oleum when the contacting operation is carried out under the same operating conditions in accordance with the prior art, i. e. in the absence of HN03.

Further, we have observed that under the conditions prevailing in the oleum-S03 contacting zone, the presence of suitable amounts of HNOs reduces the freezing points of the oleumA liquors. However, in current methods for making oleum, it is not practical especially during the summer months to cool the liquors being processed to less than say 'l5-80 F., i. e., the temperature of readily available cooling water. Thus, while presence of HNO3 in the oleum liquor in the contacting zone may lower the freezing point of some oleums down to as low as say minus 10 F., in view of the mentioned l5-80 F. practical operating temperature, we note that increased S03 absorption efficiency, arising out of lowered freezing points! and consequent permissibly lower operating temperatures, although appreciable is relatively incidental. Hence from viewpoint of summer operation, the outstanding advantages afforded by the invention are attributable mostly to our discoveries as to the effect of the presence of certain quantities of HN03 in reducing the per cent S03 by volume in vapor over the oleum, the substantial extent of which advantages will subsequently appear. However, in plants Where refrigerated cooling water is available, and in winter when cooling water may be held at say l0-50 F., substantially increased S03 absorption eiciency may be had by reason of lowered freezing points.

During development work resulting in the invention difficulties were encountered, even at relatively high operating temperatures, in connection with formation of a solid phase compound, a complex combination of S03 and N205, which solid plugged up the oleum producing tower and associated oleum liquor circulating system. To prevent formation of solid, We found that the quantity of HNO3 present in the oleum liquor throughout the oleum producing system, i. e. in the oleum production tower and in all liquor iiowing thru the cooler and piping associated with the tower, should be such as to provide in the system an oleum liquor containing some but not more HN03 than is necessary to form an oleum-nitric acid liquor having a total S03 to N205 mol ratio of not less than 13S0s to one N205. As employed herein, total S03 is intended to include the total of combined and free S03. This feature is applicable to all modifications of the invention regardless of the strength of the oleum manufactured and is the controlling factor with respect to prevention of formation of solids either during processing or in the ultimate product. To avoid likelihood of forming solids and to provide operating conditions which need not be watched too closely in practice, it is preferred to regulate the HNOa content of the oleum liquors so as to provide therein an S03 to N205 mol ratio of not less than 20:1. It will be understood that in the manufacture of say 20%, 30%, 40%, 50%, or 60% oleum, as the strength of the oleum produced increases, even though the SOszNzOs mol ratio is held at not less than 13:1 or at any other higher given ratio, the permissible amount .by weight of basis HNO3 in the liquor increases though not at a relatively rapid rate'. For example, in case of a 20% oleum to which has been added sufficient nitric acid in the form of 100% HNOs to bring the S03:N205 mol ratio down to 13:1, it is noted that the resulting liquor contains about 9.3% by weight of HNOs. 0n the other hand, in the circumstance in which suicient 100% I-IN03 has been added to a 65% oleum to bring the S03:N205 ratio down to 13:1, the resulting liquor would contain about 10.1% by weight of HN03. Ordinarily, we find no advantage in operating with liquors containing more than about 99.3% by Weight of HNOs. Further, while liquors containing any appreciable amount of HN03 may be utilized, it is preferred to maintain in the liquors circulating thru the system at least 1% by weight of HN03. In the manufacture of the more usual commercial grades of oleum of strengths ranging from Say 20 to 60% and upward, preferred procedure is such that the liquors circulating thru the system contain from .4 to about 6%V by weight of HNOa. It is noted that in practice of all modifications of the invention in which the oleum constituent of the liquor at any point in the oleum production system contains at least some free S03 and not more than 6% by weight of HNO3, the SO32N2O5 mol ratio of such liquor is not less than about 20:1, and under these conditions the SOazNzOs mol ratio is inherently maintained higher than the permissible minimum 13:1 ratio above described. Thus in the manufacture of products of all oleum strengths, when it is desired to operate, as is preferred, so that the SO3IN2O5 mol ratio in all liquors in the oleum production system is not less than 20:1, such liquors should containnot more than 6% by Weight of HNO3.

We have found thatJ under some circumstances the nitric acid in the oleum production Zone developed vapor pressure sufficient so that some HNO; as Suc-h exited the Aoleum production Zone along with unabsorbed S03 and passed into the nal sulfuric acid production tower. 1n order to prevent exit of nitric acid from the oleum production zone, and to avoid operating diiiiculties with respect to equipment corrosion and final removal of all HNOS and SOs from the gas stream before discharge to the atmosphere, we have discovered that certain further conditions should be maintained in the oleum production zone. We find that in order to prevent the presence of HNOS in the gas stream leaving the oleum production zone, the contacting operation in the oleum production zone should be such that the oleum liquor, at the point of last contact of S03 gas with liquor, has a total acidity of not less than 104. For example, in a countereurrent contacting operation, formation of the oleum-nitric acid liquor should be such that the liquor as fed to the top of the oleum production tower has a total acidity of not less than 104. By total acidity of a liquor is meant the numerical sum of (l) the percent by weight of 100% HNO3 plus (2) the per cent by weight of equivalent HzzSOi, contained in the liquor. For example, straight 20% oleum (before addition of any nitric acid) contains 104.5% equivalent HzSOi and obviously no H1103. Hence the total acidity as referred to herein, of straight 20% oleum is 104.5 plus Zero equals 104.5. On the other hand, a liquor madeby addition of enough 100% HNO3 to straight 20% oleum to form a liquor containing 1% `by weight of HNOa-contains 103.4% equivalent H2SO4 and 1% by weight of 100% HNOS. Thus the total acidity of such a liquor is 103.4 plus one equals 104.4. As well known in this art, per cent equivalent sulfuric acid is the value obtained if all the S03 in an oleum liquor were combined with H2O to form H2SO4.

In practice of this invention, the HNO3 may be introduced into the oleum liquors in any suitable form, e. g. as 100% HNOa, as a mixed acid containing say 84% HNOS, 12% H2SO4 and balance Water, or as concentrated nitric acid having an HNOS strength of about 93%, the latter being a readily available and preferred form. Weaker nitric acid may also be used if desired.

With reference to the making up of the oleumnitric acid liquor to be fed into the oleum production zone, it will be understood that the addition of a given amount of HNOs to an oleum of given strength will form an oleum-nitric acid liquor having a given total acidity. For example, in the case of a 40% oleum (containing 109% by weight equivalent H2504) to which has been added suilicient nitric acid in the form of 100% HNO3 to incorporate 6% by weight of HNOs in the resulting oleum-nitric acid liquor, such resulting liquor would have an SO3cN2O5 ratio of 21.9:1, and contain (l) 6% by weight of 100% HNOS and (2) 102.5% by weight of equivalent H2SO4; and in accordance with the above stated definition the total acidity of the liquor would be 6 plus 102.5 equals 108.5. On the other hand, if in a similar situation there were added to 40% oleum sufficient nitric acid in the form of 93% strength nitric acid to incorporate 6% by weight of HNO3 in the resulting oleum-nitric acid liquor, such resulting liquor would have an SO3zN2O5 ratio of 213:1; and contain (1) 6% by weight of HNOS and (2) 101.9% by weight of equivalent HzSOLi. Hence, total acidity would be 6.0 plus 101.9 equals 107.9. Accordingly, it will be understood :that in the circumstances in which thenitric acid is introduced into the system in a form containing H2O, such H2O acts as a diluent of the total mass, lessens the per cent by weight of 100% HNO3 and also the per cent by weight of equivalent H2SO4, and correspondingly reduces total acidity, the extent of total acidity reduction being dependent upon the amount of H2O contained in the nitric acid. 1t is noted that in practice of all modifications of the invention in which the liquor at any point in the oleum production system has a total acidity of not less than 104 and contains not more than 6% by weight of HNO3, the SOgNzOs mol ratio of such liquor is not less than about 21:1, and hence under these conditions the SO3:1\T2O5 mol ratio is automatically maintained higher than the permissible minimum13 :1 ratio above described.

In View of the foregoing disclosures and explanatory comments, the various factors to be taken into consideration in making up and using oleum-nitric acid liquors according to this invention willfbe apparent to those skilled in the art.

One embodiment of the invention may be described in connection with Fig. 2 of the drawing. To start operation, tower I0 and its liquor circulating system may be filled with strong say 99% sulfuric acid, and oleum tower II and its liquor circulating system may be filled with intermediate strength e. g. 20% oleum. Representative S03 strength and temperature of incoming sulfur trioxide gas, temperature of liquor kentering Yand leaving tower l i, and temperature of the liquor fed into the top of tower I9 may be as indicated bythe legends o-n the drawing. Towers Il and I0 may be operated as known in the art until the strength of the oleum in tower I I, cooler l5 and associated piping is raised to 40 strength, i. e. 109% equivalent H2SO4. For illustrative purposes it may be assumed that the extraneous nitric acid employed is of 100% strength. Sunicient 100% nitric acid is introduced by way of cooler I5 to incorporate, in the liquor in the system of tower II, 5% by weight of 100% HNO3. Thus, there is formed in cooler I5, an oleumnitric acid liquor having an SC3-N205 ratio of 26.611, containing 5% by weight of 100% NHOs. and 103.6% by weight or" equivalent H2SO4, such liquor having a total acidity of 5 plus 103.6 equals 108.6. Circulation of liquor over tower II is started, composition and temperature of incoming S03 gas, temperature of liquor entering vand leaving tower II being maintained as before. Liquor fed to the top of tower II is contacted countercurrent. with gas containing S03, and the per cent free S03 in the liquor is increased, the amount of such increase being dependent upon particular design ofv the tower and the rates of liquor and gas contact. In a tower of more or less usual design, increase of free S03 in the oleum constituent of liquor during one pass thru the tower may be say 2%. In this situation the oleum constituent of liquor discharged from the bottom of tower II contains 42% by weight free S03, and the mixture would contain 4.8% by weight of 100% HNO: and 104.2% by weight of equivalent H2804, such liquor having a total acidity of 4.8 plus 104.2 equals 109. The "make is drawn out of the system thru pipe 34, and suflicient strong sulfuric acid from tower I8 and enough 100% nitric acid are run into cooler I5 -to maintain the desired composition of the absorbing liquor under the particular conditions -of operation.

When operating in this manner, i. e. keeping the liquor-gas equilibrium temperature at the top of tower I I at about 105 F. and using an 8.1% S03 gas, the oleum tower exit gas contains about 1.4% S03 by volume, this 1.4% value comparing with the 4.8% S03 value shown on Fig. 1 indicating conditions existing when producing 40% oleum and operating in accordance with the prior art. In the operation of Fig. 1, the S03 absorption eiiiciency in tower I is only 42% whereas in the operation of Fig. 2, S03 absorption eiiiciency eiected in tower Il is 83.9%, this marked increase of S03 absorption eiiiciency arising from our discoveries as to the effect of the presence of HN03 in the liquor in reducing the per cent S03 by volume in vapor over the oleum containing such nitric acid.

The advantages of the invention may be further appreciated from consideration of the curves shown on Figs. 4, and 6. Fig 4 shows per cent free S03 values (oleum strengths as such) and the per cent S03 by volume in vapor leaving such oleums. Curves 50, 5I and 52 indicate absorption operations carried out at temperatures of 130 F.,

110 F. and 86 F. respectively, the oleums containing no nitric acid. Curves 53, 54 and 55 show absorption operations carried out at temperatures of 130 F., 110 F., and 86 F. respectively, the oleums in these instances containing 4 5-55% by weight of 100% I-IN03. The strikingly marked decrease of per cent S03 by volume in vapors leaving the oleums containing nitric acid is obvious from a comparison of curves 50, 5| and 52 on the one hand and curves 53, 54 and 55 on the other.

Fig. 5 indicates absorption eiciencies at various temperatures. Curves 51, 58, 59, 60 and 5I of Fig. 5 show that by the use of absorbing liquors made up by addition of 40% oleum of 4.5-5.5% I-IN03 (introduced as 100% HN03) and sulfur trioXide gases containing 7%, 8.1%, 9.1%, 10.2%, and 11.3% S03 by volume respectively, absorption eiiiciencies at representative operating temperature of 105 F. vary from about 80 to 88%.

Fig. 6 represents the results obtained by the addition of variable amounts of nitric acid to 50% oleum, absorption being carried out at 86 F. It will be noted that as the per cent nitric acid increases the per cent S03 by volume in the vapor leaving the oleums decreases markedly.

Fig. 3 of the drawing indicates an operation in which the liquor-gas equilibrium temperatures at the top of tower I2 are maintained at 80 F. In all other respects, the operation of tower I2 is the same as the operation of tower II of Fig. 2. In the procedure of Fig. 3, it is possible to lower minimum operating temperature to 80 F. because the presence of nitric acid lowers the freezing point of the oleum to less than 70 F. As will be seen from inspection of curves 51-6I of Fig. 5, in the process of Fig. 3, at temperature of about F., S03 absorption efficiency is about and the process becomes a one tower system in which a second tower for cleanup of S03 is unnecessary.

The above principles oi the invention apply to the manufacture of all grades of oleum, and by oleum is meant a liquor containing less H20 than that needed to combine with total S03 of the liquor to form H2S04. It will be understood that in practice of the invention, aside from the above described features relating to the nature of the oleum-nitric acid liquors which are contacted with S03 gas, other operating factors such as design of the oleum and sulfuric acid production towers and rates of gas and liquor ow thru the towers are the same as known in the art, except that in cases where liquor cooling facilities permit, the oleum production towers may be operated at lower minimum temperatures in accordance with the above indicated decreased freezing points of the oleum liquors.

The herein described reduction of per cent of S03 by volume in vapor over oleum is of particular value in connection with manufacture of relatively high strength oleums. Accordingly, in the more desirable embodiments of the invention, it is preferred to operate in such a way that the product liquor drawn ofi from the process contains 4-6% by weight of HN03 and not less than 30% free S03.

We claim:

l. In the manufacture of an oleum-nitric acid mixture the improvement comprising passing a gas containing S03 through an oleum-nitric acid liquor containing at least 1% by weight of HN03, the amount of said HN03 being such that the total S03 to N305 mol ratio is not less than 13:1, and carrying out such contacting operation under conditions to absorb S03 in said liquor and to maintain in said liquor an S03 to N305 mol ratio of not less than 13 l, whereby the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone is substantially lower than would exist under the same conditions but in the absence of I-IN03, and recovering from the contacting operation a liquor of increased total S03 strength.

2. In the manufacture of an oleum-nitric acid mixture the improvement comprising passing a gas containing S03 through an oleum-nitric acid liquor containing at least 1% and not more than 6% by weight of I-IN03, and carrying out such contacting operation under conditions to absorb S03 in said liquor and to maintain in said liquor not more than 6% by weight of HN03, whereby the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone is substantially lower than would exist under the same conditions but in the absence of HN03, and recovering from the contacting operation of liquor of increased total S03 strength.

3. In the manufacture of an oleum-nitric acid mixture the improvement comprising forming an oleum-nitric acid liquor containing 4-6 by weight of I-IN03 passing a gas containing S03 through said liquor in a gas-liquor contacting zone under conditions to absorb S03 in said liquor and to maintain in said liquor not more than 6% by weight of HN03, whereby the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone is substantially lower than would exist under the same conditions but in the absence of HN03, and recovering from said zone a liquor of increased total S03 strength.

4. In the manufacture of an oleum-nitric acid mixture the improvement comprising passing a gas containing S03 through an oleum-nitric acid liquor containing at least 1% by weight of I-IN03, the amount. of said HN03 being such that the total S03 to N305 mol ratio is not less than 13:1, the said liquor having at the point of last contact of S03 gas with such liquor a total acidity of not less than 104, and carrying out such contacting operation under conditions to absorb S03 in said liquor and to maintain in said liquor an S03 t0 N205 mol ratio of not less than 13:1, whereby the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone is substantially lower than would exist under the same conditions but in the absence of HN03, and recovering from the contact operation a liquor of increased total S03 strength.

5. In the manufacture of an oleum-nitric acid mixture, the improvement comprising continuously forming an oleum-nitric acid liquor (a) containing at least 1% by weight of I-IN03, the amount of said HNO3 being such that the total S03 to N205 mol ratio is not less than 13:1, and (b) having a total acidity of not less than 104, continuously introducing said liquor in a gasliquor contacting zone, countercurrently contacting said liquor therein with gas containing S03 under conditions to absorb S03 in said liquor and to maintain in said liquor an S03 to N305 mol ratio of not less than 13:1. whereby there is effected during the contacting operation a substantial lowering of the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone as compared with the S03 vapor pressure existing under the same conditions but in the absence of HNOa, and continuously withdrawing liquor of increased total S03 strength from said zone.

6. In the manufacture of an oleum-nitric acid mixture, the improvement comprising continuously forming an oleum-nitric acid liquor (a) containing at least 1% and not more than 6% by weight of I-IN03, and (b) having a total acidity of not less than 104, continuously introducing said liquor` into a gas-liquor contacting zone, countercurrently contacting said liquor therein with gas containing S03 under conditions to absorb S03 in said liquor and to maintain in said liquor not more than 6% by weight of HNOa, whereby there is effected during the contacting operation a substantial lowering of the vapor pressure of S03 over the oleum-nitric acid liquor in the contact zone as compared with the S03 vapor pressure existing under the same conditions but in the absence of HN03, continuously withdrawing liquor of increased total S03 strength from said Zone and discharging therefrom gas containing residual S03, and contacting such gas with strong sulfuric acid under conditions to absorb substantially all of said residual HENRY F. MERRIAM.

TOM CUMMINGS.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 884,035 Nield Apr. 7, 1908 1,047,576 Schultze Dec. 17, 1912 1,291,306 Weber Jan. 14, 1919 1,605,004 Shapleigh -f Nov. 2, 1926 1,896,287 Clark Feb. 7, 1933 2,047,546 Clark July 14,1936 2,139,721 Carl Dec. 13, 1938 

6. IN THE MANUFACTURE OF AN OLEUM-NITRIC ACID MIXTURE, THE IMPROVEMENT COMPRISING CONTINUOUSLY FORMING AN OLEUM-NITRIC ACID LIQUOR (A) CONTAINING AT LEAST 1% AND NOT MORE THAN 6% BY WEIGHT OF HNO3, AND (B) HAVING A TOTAL ACIDITY OF NOT LESS THAN 104, CONTINUOUSLY INTRODUCING SAID LIQUOR INTO A GAS-LIQUOR CONTACTING ZONE, COUNTERCURRENTLY CONTACTING SAID LIQUOR THEREIN WITH GAS CONTAINING SO3 UNDER CONDITIONS TO ABSORB SO3 IN SAID LIQUOR AND TO MAINTAIN IN SAID LIQUOR NOT MORE THAN 6% BY WEIGHT OF HNO3, WHEREBY THERE IS EFFECTED DURING THE CONTACTING OPERATION A SUBSTANTIAL LOWERING OF THE VAPOR PRESSURE OF SO3 OVER THE OLEUM-NITRIC ACID LIQUOR IN THE CONTACT ZONE AS COMPARED WITH THE SO3 VAPOR PRESSURE EXISTING UNDER THE SAME CONDITIONS BUT IN THE ABSENCE OF HNO3, CONTINUOUSLY WITHDRAWING LIQUOR OF INCREASED TOTAL SO3 STRENGTH FROM SAID ZONE AND DISCHARGING THEREFROM GAS CONTAINING RESIDUAL SO3, AND CONTACTING SUCH GAS WITH STRONG SULFURIC ACID UNDER CONDITIONS TO ABSORB SUBSTANTIALLY ALL OF SAID RESIDUAL SO3. 